1. Field of the Invention
The present invention relates to an external antenna device for receiving short-wave broadcasts.
2. Description of the Related Art
Portable short-wave receivers are usually equipped with a rod antenna, which, however, is insufficient by itself in reception sensitivity. To enhance reception sensitivity some short-wave receivers are disposed to permit connection of an external antenna.
Known examples of such external antennas include a wire antenna and a combination of another rod antenna with a high-frequency amplifier. A wire antenna, which may be either a simple wire or a lead supplying the signals it has received as they are to the receiver, can achieve the required level of sensitivity if it is long enough. An active antenna combined with a high-frequency amplifier can also achieve the required level of sensitivity.
However, because a wire antenna is non-directional, it supplies the receiver with unnecessary radio wave signals, too, and accordingly is disadvantageous respect to interference characteristics.
An active rod antenna, if it is made compact by reducing the length, may become inadequate in sensitivity even if it is combined with a high-frequency amplifier or, if it is made long enough to be sufficiently sensitive, its overall size will inevitably increase. Furthermore, the relatively low directivity makes the rod antenna also disadvantageous in interference characteristics and other respects.
The present invention is intended to solve the problems noted above.